Linux RHEL 5 configuring multipathing with DM-Multipath

In my example, I am connected to an IBM DS5020 from 2 IBM blade servers. The blade chasis is equipped with 2 Brocade 4GB SAN switch modules. Each blade server has two access paths, provided the server is installed with 2 Qlogic HBA modules.

Currently, I have configured 2 Logical Drives to be presented to each blade server. Both have access to ASM (LUN0), and independently blade A has access to ORA1, blade B access to ORA2. By default, multipathd isn’t started.

The procedure below will walk through the configuration of multipathing.

Here’s the output of fdisk to show what the disks are seen by the system — notice the 4 paths to 50/100GB devices:
# fdisk -l

Setting up DM-Multipath
Edit /etc/multipath.conf and comment out the ‘blacklist’ section
#blacklist {
# devnode “*”
#}
Save the file and execute the following commands:
modprobe dm-multipath
service multipathd start
multipath -v3 # will print out all multipathed paths
chkconfig multipathd on

Since the value of “user_friendly_name” is set to “yes” in multipath.conf, the multipath devices will be created as /dev/mapper/mpathn.
# ls -l /dev/mapper/
control mpath1 mpath2

From above, we see that /dev/sda is the internal disk and should be ignored from the configuration. Edit /etc/multipath.conf again and add the following stanza:
blacklist {
wwid 3600508e000000000b9cfe9bbf3879804
}

Note, if your device does not show up, it may be because the default supported devices does not include your storage array. Additional ones can be added by following viewing /usr/share/doc/device-mapper-multipathd-0.4.7/multipath.conf.annotated.

I’ve updated my steps to include showing how to get the WWID, using mulitpath command.

For part2 of your question, the commands I listed on the bottom are reference commands for when you need to apply those changes. So, you are right mpath0 in this case isn’t associated with what I’ve done above. When you resize a device, you put the correct mpathX device you are going to resize. In my examples above, it would be either mpath1 or mpath2.